Acrylic latexes are used in many products, and are known. Their ingredients, synthesis methods, and results characterization can have an impact on their particular end-use, particularly in the area of coating compositions such as paints and stains. The patent literature discloses latex compositions of acrylic polymers and copolymers, many for use in the coatings art.
For example, U.S. Pat. No. 5,990,228 and U.S. Patent Application Publication No. 2005/0014883 A1 both disclose providing pre-polymerized “seed” latex particles to a reaction vessel and only then further conducting a polymerization in their presence. As disclosed in the '228 patent, aqueous coating compositions containing at least two polymer components provide adhesion and improved durability, as measured in dried coatings made from the aqueous compositions by improving gloss retention or dirt pickup resistance. Specifically, Comparative Example 1 in the '228 patent discloses the use of a pre-polymerized latex core of 60 nm particulate poly(butyl acrylate-co-methyl methacrylate-co-methacrylic acid), upon which was further polymerized two layers of poly(butyl acrylate-co-methyl methacrylate-co-acrylic acid) of the same composition. Furthermore, all of the Examples in this patent disclose compositions that are polymerized in the presence of these pre-formed 60 nm core latex particles. In addition, the '228 patent discloses acrylic copolymers having very low proportions of acid monomers (e.g., methacrylic acid and acrylic acid).
In coating applications, the acrylic latexes are sometimes reacted with a crosslinking component, such as an epoxy compound, to form a crosslinked coating. Because epoxies are reactive with many different types of functional groups, such as hydroxyls, carboxylic acids, amines, etc., there has been a wide variety of different compositions of acrylic monomers used to attain various properties, such as increased latex stability, acceptable curing behavior with epoxies, etc. Several publications in the patent literature focus on core-shell type acrylic particles having hydroxyl functionality specifically in the core of the particles. Most of these publications also require a substantial glass transition temperature difference between the core polymer and the shell polymer.
For example, International Publication No. WO 94/04581 discloses waterborne latices containing core-shell acrylic particles that are adapted for spray application to form coatings that impart impact resistance, sag resistance, and solvent popping resistance. This publication teaches that the core contains 50-90 wt % acrylic and methacrylic monomers and 1-20 wt % hydroxy-functional monomers so as to form a polymer having a Tg below 0° C. This publication also teaches that the shell contains 40-90 wt % acrylic and methacrylic monomers, 5-20 wt % acid-containing monomers, and 1-20 wt % hydroxy-functional monomers so as to form a polymer having a Tg above 20° C.
In addition, European Patent Application No. EP 0 614 919 A1 describes a latex composition containing core-shell type polymer particles in which the core is made from 50-90 wt % C1-C4 alkanoate monomers and 10-50 wt % monomers being vinyl esters of tertiary saturated carboxylic acids so as to form a polymer having a Tg of at least 10° C., and in which the shell is made from 10-50 wt % C1-C4 alkanoate monomers and 50-90 wt % monomers being vinyl esters of tertiary saturated carboxylic acids so as to form a polymer having a Tg below 10° C. In addition, the application requires a difference between the Tg values of the core and the shell of at least 5° C.
Further, U.S. Pat. No. 6,723,779 describes a low-temperature film-forming latex based on hydrophobic polymers in which crosslinking monomers, such as dienes, divinyl acrylates, polyvinyl compounds, and the like, are used so that the composition is self-curing. This patent also teaches that the latex polymers have a core-shell morphology in which 70-90 wt % of the particle is the core polymer having a Tg below 20° C. and 10-30 wt % of the particle is the shell polymer having a Tg above 50° C.
As shown, for example, in European Patent Application No. EP 0 614 919 A1 and in U.S. Pat. No. 6,723,779 above, other ways to attain various properties, such as stability, corrosion resistance, weatherability, and the like, in the latex particles and the coatings made therefrom include copolymerizing the acrylic monomers with different types of comonomers. One such popular comonomer is acrylamide. However, the use of acrylamides may cause health and disposal issues, and it is desirable to find other ways to improve these properties without increasing the toxicity of the latex particles and the products made therefrom.
Another approach to controlling molecular weight is to establish a living ionic polymerization, as disclosed in U.S. Pat. No. 5,548,043. Although living polymerizations are typically only used to polymerize vinyl monomers having an increased ability to delocalize the ionic charge on the living end of the polymerizing molecule (e.g., conjugated dienes and/or styrenic monomers), this patent generally teaches the formation of many different types of low-melt toner polymer resins that have bimodal molecular weight distributions. The bimodality disclosed in the aforementioned patent is accomplished by selective termination of the living polymerization, e.g., by introducing air, resulting in a high and a low molecular weight peak. However, the living ionic polymerization approach is not typically viable for predominantly acrylic-based polymers and is typically not performed in an emulsion or latex medium.
Thus, there remains a need for a multimodal acrylic-based polymer latex which exhibits desirable performance properties without increased toxicity.